Perinatal mortality rates can differ up to 50% depending on the definitions used to include a stillbirth. The criteria can be a minimum gestational age , a minimum birth weight, or a combination of both.

Previously, the World Health Organisation (WHO) - in the context of ICD9 – considered the birth weight as the first criteria to include a foetal death in the statistics : for national use, the WHO recommended to include in theperinatal statistics"… all live and death born children with a minimum birth weight of 500 g, or if this information was unknown, those with a pregnancy duration of minimum 22 weeks."For international use, the WHO recommended to include : "… all live and death born children with a minimum birth weight of 1000 g, or if this information was unknown, those with a pregnancy duration of minimum 28 weeks ".

More recent definitions give priority to the gestational age as first criteria to register fetal deaths.

EURO-PERISTAT:EURO-PERISTAT defines stillbirth as fetal death at or after 22 completed weeks of gestation.